An imaging method is generally used to inspect objects or substances in a non-destructive manner. Two methods, i.e., an image detection method using a continuous output light source and an image detection method using a spectroscopic method are mainstream imaging methods. These methods have advantages and disadvantages, but the image detection method using a continuous output light source is widely used in fields requiring relatively high power such as a transmission image.
A terahertz wave is widely used in the field of qualitatively checking for a hidden object or substance in a non-destructive manner due to various excellent properties such as transmissivity with respect to materials, possibility of qualitative checking, safety to a living body, and the like.
Accordingly, the terahertz wave has recently been utilized in various fields such as a search device in an airport or a security facility, a quality inspection device in a food or pharmaceutical company, a semiconductor inspection device, an engineering plastic inspection device, and the like.
The use of terahertz waves in production sites has increased, and much improvement has been made by continuous research in terms of major performance indices such as detection resolution, detection rate, detection area, and the like.
In the past, in order to obtain a terahertz wave transmission image, only one lens was used to concentrate a terahertz wave which is radiated after being transmitted through an object. In this case, if an apical angle of an axicon lens forming a Bessel beam is regulated to be small so as to make a beam size of the terahertz wave focused on an inspection target object be less than a wavelength, a terahertz Bessel beam passing through the inspection target object is radiated at a large angle and is not entirely concentrated to a detection unit. Thus, light concentration characteristics significantly deteriorates and a signal per noise ratio (SNR) of an inspection device is drastically degraded, and thus, a normal image cannot be obtained.
Further, it is difficult to obtain a clear image of a transparent inspection target object. Therefore, there is a need for research and development of a method for enhancing the contrast of a transparent inspection target object with little loss of a terahertz wave.
In addition, there is a problem in that a high resolution image cannot be obtained because a depth of focus of the Bessel beam does not reach an end portion of the inspection target object.
Further, if the inspection target object contains a large amount of water, a proportion of a terahertz wave transmitted through the inspection target object is drastically lowered due to the properties of terahertz waves being easily absorbed by moisture. As a result, the detecting unit cannot correctly inspect the inspection target object because a signal of the detected terahertz wave is weak.
The related art of the present invention is disclosed in Korean Patent Registration No. 10-1392311.